This invention relates to a variable delivery fluid pump and an electro-hydraulic control circuit therefor, and more particularly, to a fluid pump for use with a fuel injection system or other hydraulic system for an internal combustion engine.
In a common rail fuel injection system, high pressure fluid is supplied to the injectors from a high pressure fluid accumulator or manifold, which is referred to as a rail. To permit variation of the fluid pressure supplied to injectors from the rail, it is desirable to vary the delivery of fluid to the rail from one or more fluid supply pumps. Known common rail systems typically rely on either a single fluid pump that supplies fluid to the rail or a plurality of smaller displacement pumps that each supplies fluid to the rail. The volume and rate of fluid delivery to the rail has been varied in the past by providing a rail pressure control valve that spills a portion of the delivery from a fixed delivery pump to maintain the desired rail pressure. Both high pressure and low pressure common rail systems are known in the art. In high pressure common rail systems, high pressure fuel is supplied from the rail to electrically-controlled injection nozzles. In a low pressure common rail, an actuation fluid such as fuel or engine lube oil is supplied from the rail to unit injectors, whereby the actuation fluid is used to drive a fuel pressurization plunger that pressurizes the fuel to injection pressure prior to or during each injection event.
Variable delivery pumps are well known in the art and are typically more efficient for common rail fuel systems than a fixed delivery actuation fluid pump, since only the volume of fluid need to attain the desired rail pressure must be pressurized. For example, variable delivery has been achieved from an axial piston pump, e.g. a pump wherein one or more pistons are reciprocated by rotation of an angled swash plate, by varying the angle of the swash plate and thus varying the displacement of the pump. In such a pump, the swash plate is referred to as a xe2x80x9cwobble platexe2x80x9d. Variable delivery has also been achieved in fixed displacement, axial piston pumps by a technique known as sleeve metering, in which each piston is provided with a vent port that is selectively closed by a sleeve during part of the piston stroke to vary the effective pumping portion of the piston stroke. An example of such a sleeve-metering pump is illustrated in commonly-owned U.S. Pat. No. 6,035,828.
While known variable delivery pumps are suitable for many purposes, known design are not always well suited for use with modem common rail fuel systems, which require fluid delivery to the rail to be varied with high precision and with rapid response times measured in microseconds. In addition, known variable delivery pumps are typically complex, may be costly, and are subject to mechanical failure. Moreover, in some known pumps such as the pump shown in commonly-owned U.S. Pat. No. 6,035,828, the relative positioning of the pumping pistons and the metering sleeves is controlled by way of an electro-hydraulic control circuit which receives high pressure fluid directly from the delivery gallery of the pump at high pressure and selectively spills that control fluid via an electrically-operable control valve. While pumps such as the one illustrated in U.S. Pat. No. 6,035,828, have performed well, room for improvement exists due the current need for small, high-precision passages and valve elements in the prior art as a result of the high fluid pressures present in the control circuit.
This invention is directed toward overcoming one or more of the problems described above.
In one aspect of this invention, a hydraulic pump system comprises a variable delivery, sleeve-metered pump having a plurality of pumping pistons and associated metering sleeves. The pumping pistons deliver pressurized fluid to a high pressure area at a pressure at least equal to a first pressure. An electrically-operated, hydraulic control circuit is operable to control the delivery of pressurized fluid from said pump by controlling the relative position between the pistons and their associated metering sleeves. The control circuit is in fluid communication with the high pressure area and has a pressure reducer to reduce pressure of fluid entering the control circuit to a control circuit pressure less than the first pressure.
In another aspect of this invention, the control circuit comprises a pressure reducing valve having an inlet in fluid communication with a high pressure area of the pump and having a valve outlet. The pressure reducing valve reduces the pressure of control fluid entering the control circuit to a predetermined control circuit pressure. A movable control member has a first control surface and a second control surface opposed with the first control surface, movement of the control member changing the relative positioning between the pumping pistons and their associated sleeves. A control line is in fluid communication with the pressure reducing valve outlet and has a first, relatively unrestricted passageway through which fluid pressure is applied to the first control surface and a second, relatively-restricted passageway through which fluid pressure is applied to the second control surface. An electrically operated control valve is fluidly connected with the control line to selectively control the relative fluid pressures applied to the first and second control surfaces.
In yet another aspect of this invention, the control circuit comprises a pressure reducing valve having an inlet in fluid communication with a high pressure area of the pump and having a valve outlet. The pressure reducing valve reduces the pressure of control fluid entering the valve to a predetermined control circuit pressure. A movable control member has a control surface, and movement of the control member changes the relative positioning between the pumping pistons and their associated sleeves. A control line is in fluid communication with the pressure reducing valve outlet and has a restricted passageway through which fluid pressure is applied to the control surface. A bias member applies force to the control member in a direction opposite the fluid pressure applied to the control surface. An electrically operated control valve is fluidly connected with the control line to selectively control the fluid pressure applied to the control surface.
In still another aspect of this invention, a method of controlling the delivery of pressurized fluid from a variable delivery, sleeve-metered pump is provided. The pump comprises a plurality of pumping piston and associated metering sleeves. The method comprising reciprocating the pistons to thereby deliver pressurized fluid to a high pressure area of the pump at pressure at least equal to a first pressure, delivering a portion of the pressurized fluid to a control circuit operable to selectively control the relative position between the pistons and their associated metering sleeves, reducing the pressure of the fluid delivered to the control circuit to a pressure less than the first pressure, and using the reduced-pressure fluid to control the relative position between the pistons and their associated metering sleeves, thereby controlling the delivery of pressurized fluid from the pump.